A hybrid electric vehicle (HEV) powertrain may include two power sources for delivering driving power to vehicle traction wheels. In general, these power sources may be divided into two broad categories: mechanical and electrical. A mechanical power source may include, for example, an internal combustion engine that outputs mechanical power to the vehicle wheels through one or more sets of transmission gears. An electrical power source may include one or more electric machines, such as an electric motor or a motor/generator. The electric motor may receive electrical power from another generator, a battery, or other source of electrical power.
The two power sources must be integrated to work together seamlessly to meet a driver's demand for power within system power constraints while optimizing total powertrain system efficiency and performance. This requires a coordination of control of the two power sources. Because this type of arrangement provides multiple power flow paths to the vehicle wheels, the engine speed can be optimized for the maximum system efficiency for a given driver demand for power. However, when the engine is commanded to operate at a power corresponding to maximum system efficiency, the power surplus or deficit is absorbed by the battery. Because of battery capacity limits and/or constraints combined with the need to balance the battery state of charge (SOC), it may not be feasible to always operate the engine at a power corresponding to maximum system efficiency for a given driver demand.
Optimizing HEV powertrain operation remains a difficult challenge. HEV energy management is the core strategy for efficient powertrain operation. Usually for a given battery power request, the rest of the energy management is a static optimization problem. The determination of battery power request is often heuristic and thus sub-optimal, resulting in sub-optimal fuel economy. As such, there exists a need for more accurate and robust methods for determining the battery power request for a given driver demand that balances battery SOC and allows the engine to operate at a power that increases system efficiency as much as possible during a drive cycle.